Loop antenna and design method for loop antenna
Even when at least one of a capacitor (C1) connected to a main loop and a capacitor (C2) connected to an amplification loop cannot be set to an optimal value, a current value of a current (I2) flowing on the amplification loop can be made sufficiently large by setting the capacitors (C1, C2) based on any of an optimal C2 curved line, an optimal C1 curved line, and an optimal C1 straight line that pass through an optimal point (C1opt, C2opt) of the capacitors (C1, C2) and extend along a ridge of contour lines each joining the points where the magnitude of the current (I2) is equal on a diagram showing a relation of values of the capacitors (C1, C2) with the magnitude of the current (I2).
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The present invention relates to a loop antenna that can contribute to expansion of a coverage for a wireless system using a magnetic field.
BACKGROUND ARTIn recent years, a service linked to an intention and an action of a user while purposely restricting an authentication area has been provided by using an authentication system adopting wireless communication techniques such as the near field communication (NFC) (Patent Documents 1 to 3). A loop antenna (a coil) is employed when forming the authentication area by using a magnetic field. A current applied to the antenna develops spherical magnetic field distribution on a surface of the antenna. A distance decay property of the magnetic field is shaper than that of an electric wave, and therefore has an advantage that it is possible to clearly mark off a boundary of a wireless coverage.
The sharp distance decay property of the magnetic field is a drawback from the viewpoint of expanding the wireless coverage. When expanding the wireless coverage in the wireless system using the magnetic field, a large current needs to be applied to the antenna, which may lead to a significant increase in power consumption.
A method of amplifying the magnetic field by using a magnetic field resonance effect without increasing a consumption current has been proposed as a mode of solving the aforementioned problem (Patent Documents 4 and 5).
PRIOR ART DOCUMENTS Patent DocumentsPatent document 1: Japanese Patent Publication No. 5684695
Patent document 2: Japanese Patent Publication No. 5914368
Patent document 3: Japanese Patent Publication No. 5813672
Patent document 4: Japanese Patent Publication No. 6077036
Patent document 5: Japanese Patent Publication No. 6077148
SUMMARY OF THE INVENTION Problem to be Solved by the InventionIn order to amplify the magnetic field by using the resonance, capacitances of capacitors that are fitted to two tightly coupled loop antennas, respectively, need to be set to appropriate values. A variable capacitor is convenient in order to set the capacitance of such a capacitor to an optimal value. On a practical point of view, the use of a fixed capacitor is required to meet the needs for reliability and cost reduction.
However, it is difficult to set the capacitance of the fixed capacitor to the optimal value. For this reason, there has been a demand for a method of obtaining a relatively large magnetic field even if the capacitances of the capacitors do not completely match the optimal values.
The present invention has been made in view of the aforementioned circumstances and an objective thereof is to obtain a relatively large magnetic field even if a capacitance value of a capacitor attached to an antenna does not completely match an optimal value.
Means for Solving the ProblemA loop antenna according to an aspect of the present invention includes: a main loop being an open loop connected to any of a signal source and a reception circuit; an amplification loop being a closed loop having the same shape as the main loop; a first resistor connected in series to the main loop; a first capacitor connected in series to the main loop; a second resistor connected in series to the amplification loop; and a second capacitor connected in series to the amplification loop. Here, the main loop and the amplification loop have equal selfinductance. A resistance value of the first resistor is a larger value than a resistance value of the second resistor. At least one of the first capacitor and the second capacitor is a fixed capacitor. A magnitude of a current flowing on the amplification loop is expressed by using the resistance value of the first resistor, the resistance value of the second resistor, a capacitance value of the first capacitor, a capacitance value of the second capacitor, and the selfinductance. A combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor to maximize the magnitude of the current is expressed by any of an optimal curved line and an optimal straight line each of which passes through an optimal point indicated with the capacitance value of the first capacitor and the capacitance value of the second capacitor when the magnitude of the current is maximized in orthogonal coordinates adopting the capacitance value of the first capacitor and the capacitance value of the second capacitor as respective axes. The combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor is determined based on any of the optimal curved line and the optimal straight line.
Effect of the InventionAccording to the present invention, it is possible to obtain a relatively large magnetic field even if a capacitance value of a capacitor attached to an antenna does not completely match an optimal value.
An embodiment of the present invention will be described below with reference to the drawings.
The loop antenna shown in
The main loop 1 is a coil wound around a barshaped rod 3 made of either a magnetic body or an insulating body. The number of turns in the main loop 1 is at least 1 and the number of turns is 5 in the example of
The amplification loop 2 is a coil wound around the rod 3 at a position away from the main loop 1. The number of turns in the amplification loop 2 is at least 1 and the number of turns is 5 in the example of
The main loop 1 and the amplification loop 2 have the same geometric shape. Accordingly, both loops have equal selfinductance L. Here, the main loop 1 and the amplification loop 2 may be wound at the same position on the rod 3.
When an alternating current I_{1 }is supplied from the signal source 5 to the main loop 1, an alternating current I_{2 }flows on the amplification loop 2 due to mutual inductance between the main loop 1 and the amplification loop 2. If a resistance value of the resistor R_{2 }is set smaller than a resistance value of the resistor R_{1}, the magnitude of the current I_{2 }becomes larger than the magnitude of the current I_{1}. Thus, it is possible to expand an area of a magnetic field generated by the loop antenna.
While
Next, optimal values of the capacitors C_{1 }and C_{2 }for maximizing the current I_{2 }will be described.
The magnitude of the current I_{2 }relies on multiple factors including a frequency f of a signal generated by the signal source 5, the resistor R_{1}, the resistor R_{2}, the capacitor C_{1}, the capacitor C_{2}, the shape of the loop, and so forth. For this reason, it is preferable to maximize the current I_{2 }by adjusting respective values of the resistor R_{1}, the resistor R_{2}, the capacitor C_{1}, the capacitor C_{2}.
If the value of the resistor R_{2 }is smaller than the value of the resistor R_{1}, the current I_{2 }can be maximized by setting the values of the capacitors C_{1 }and C_{2 }to optimal values C_{1}^{opt }and C_{2}^{opt }defined by the following formulae (1) and (2):
where co is an angular frequency of the signal generated by the signal source 5.
The magnitude of the current I_{2 }can be easily obtained by analyzing or simulating an equivalent circuit of the loop antenna of
The optimal values C_{1}^{opt }and C_{2}^{opt }of the capacitors C_{1 }and C_{2 }defined by the following formula (3) are obtained by applying the abovementioned conditions to the formulae (1) and (2).
C_{1}^{opt}=23.3[pF],C_{2}^{opt}=230.5[pF] (3)
As shown in
However, it may not be possible to set the capacitors C_{1 }and C_{2 }to the optimal values C_{1}o^{pt }and C_{2}Op^{t }if a fixed capacitor is used for at least one of the capacitors C_{1 }and C_{2}. This embodiment seeks an optimal value with which it is possible to maximize the current I_{2 }when using the fixed capacitor for at least one of the capacitors C_{1 }and C_{2}, and determines the capacitance values of the capacitors C_{1 }and C_{2 }based on the optimal value.
A case of using the fixed capacitor for the capacitor C_{1 }and using the variable capacitor for the capacitor C_{2 }will be considered to begin with. In other words, the value of the capacitor C_{1 }cannot be fineadjusted but the value of the capacitor C_{2 }can be fineadjusted.
By analyzing the equivalent circuit of
The following formula (5) is obtained by specifically calculating and solving the formula (4) for the capacitor C_{2}.
C_{2}=f(C_{1};ω,L,R_{1}) (5)
where a function f(C; ω, L, R) is defined by the following formula (6):
A curved line expressed by the formula (5) will be hereinafter referred to as an optimal C2 curved line.
Next, a case of using the variable capacitor for the capacitor C_{1 }and using the fixed capacitor for the capacitor C_{2 }will be considered. In other words, in contrast to the aforementioned case, the value of the capacitor C_{1 }can be fineadjusted but the value of the capacitor C_{2 }cannot be fineadjusted.
In this case, an equation defined by the following formula (7) will be considered.
The following formula (8) is obtained by specifically calculating and solving the formula (7) for the capacitor C_{1}.
C_{1}=f(C_{2};ω,L,R_{2}) (8)
where the function f(C; ω, L, R) is defined by the formula (6).
A curved line expressed by the formula (8) will be hereinafter referred to as an optimal C1 curved line.
With reference to
In the meantime, with reference to
C_{2}=−C_{1}+C_{1}^{opt}(ω,L,R_{1},R_{2})+C_{2}^{opt}(ω,L,R_{1}R_{2}) (9)
A straight line expressed by the formula (9) will be hereinafter referred to as an optimal C1 straight line.
Next, a case of using the fixed capacitors for both of the capacitors C_{1 }and C_{2 }will be considered.
It is conceivable that no variable capacitors are used at all in order to achieve cost reduction. The fixed capacitors can be selected from a lineup standardized among manufacturers which ranges from E3 series to E192 series. However, the capacitors C_{1 }and C_{2 }have to be selected from the fixed capacitors having discrete values, and it is almost impossible to select the fixed capacitors that completely match the optimal values of the capacitors C_{1 }and C_{2}. In addition, it is also extremely difficult to obtain the fixed capacitors to be used for the capacitors C_{1 }and C_{2 }such that the values of the capacitors C_{1 }and C_{2 }are located on the optimal curved line or the optimal straight line described above.
Given the situation, when using the fixed capacitors for both of the capacitors C_{1 }and C_{2}, this embodiment adopts a combination (C_{1}^{0}, C_{2}^{0}) of the capacitors out of combinations of capacitor candidates, with which a distance d (C_{1}^{0}, C_{2}^{0}) from either the optimal curved line or the optimal straight line becomes shortest. The functions to represent the optimal curved lines and the optimal straight line have been given by the formulae (5), (6), (8), and (9) and it is therefore possible to obtain the distance d (C_{1}^{0}, C_{2}^{0}) therefrom. In particular, the distance from the optimal C1 straight line indicated by the formula (9) can be easily obtained by using the following formula (10):
Next, another loop antenna of this embodiment will be described.
The loop antenna shown in
The loop antenna shown in
The main loop 1 is disposed on the planar substrate made of an insulating body. The resistor R_{1 }and the capacitor C_{1 }are connected in series to the main loop 1. The main loop 1 is the open loop that includes the terminals T and T for establishing connection to the signal source 5 or the reception circuit (not shown).
The amplification loop 2 is disposed on the same planar substrate very closely to the main loop 1. The resistor R_{2 }and the capacitor C_{2 }are connected in series to the amplification loop 2. The amplification loop 2 is the closed loop that does not include any terminals.
The main loop 1 and the amplification loop 2 have the same geometric shape. Accordingly, both loops have equal selfinductance L.
When the alternating current I_{1 }is supplied from the signal source 5 to the main loop 1, the alternating current I_{2 }flows on the amplification loop 2 due to the mutual inductance between the main loop 1 and the amplification loop 2. If the resistance value of the resistor R_{2 }is set smaller than the resistance value of the resistor R_{1}, the magnitude of the current I_{2 }becomes larger than the magnitude of the current I_{1}.
Even when the fixed capacitor is used for at least one of the capacitors C_{1 }and C_{2 }in the loop antenna of
Though the amplification loop 2 is located inside the main loop 1 in
The loop antenna of
As described above, according to this embodiment, even when at least one of the capacitor C_{1 }connected to the main loop 1 and the capacitor C_{2 }connected to the amplification loop 2 cannot be set to the optimal value, a current value of the current I_{2 }flowing on the amplification loop can be made sufficiently large by setting the capacitors C_{1 }and C_{2 }based on any of the optimal C2 curved line, the optimal C1 curved line, and the optimal C1 straight line that pass through the optimal point of the capacitors C_{1 }and C_{2 }and extend along the ridge of the contour lines each joining the points where the magnitude of the current I_{2 }is equal on the diagram showing the relation of the values of the capacitors C_{1 }and C_{2 }with the magnitude of the current I_{2}. For this reason, it is possible to obtain a large magnetic field amplification effect even when an inexpensive fixed capacitor is used for at least one of the capacitors C_{1 }and C_{2}.
EXPLANATION OF THE REFERENCE NUMERALS

 1 main loop
 2 amplification loop
 3 rod
 5 signal source
Claims
1. A loop antenna comprising:
 a main loop being an open loop connected to any of a signal source and a reception circuit;
 an amplification loop being a closed loop having the same shape as the main loop;
 a first resistor connected in series to the main loop;
 a first capacitor connected in series to the main loop;
 a second resistor connected in series to the amplification loop; and
 a second capacitor connected in series to the amplification loop, wherein
 the main loop and the amplification loop have equal selfinductance,
 a resistance value of the first resistor is a larger value than a resistance value of the second resistor,
 at least one of the first capacitor and the second capacitor is a fixed capacitor,
 a magnitude of a current flowing on the amplification loop is expressed by using the resistance value of the first resistor, the resistance value of the second resistor, a capacitance value of the first capacitor, a capacitance value of the second capacitor, and the selfinductance,
 a combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor to maximize the magnitude of the current is expressed by any of an optimal curved line and an optimal straight line each of which passes through an optimal point indicated with the capacitance value of the first capacitor and the capacitance value of the second capacitor when the magnitude of the current is maximized in orthogonal coordinates adopting the capacitance value of the first capacitor and the capacitance value of the second capacitor as respective axes, and
 the combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor is determined based on any of the optimal curved line and the optimal straight line.
2. The loop antenna according to claim 1, wherein C 2 = f ( C 1; ω, L, R 1 ) f ( C; ω, L, R ):= 1 + ω 2 C { C R 2 + L ( ω 2 L C  2 ) } ω 2 L { 1  ω 2 C ( L  C R 2 ) }
 the first capacitor is the fixed capacitor, and
 the optimal curved line is expressed by a formula defined as:
 where C1 is the capacitance value of the first capacitor, C2 is the capacitance value of the second capacitor, L is the selfinductance of the main loop and the amplification loop, co is an angular frequency of a signal to be applied to the main loop, and R1 is the resistance value of the first resistor.
3. The loop antenna according to claim 1, wherein C 1 = f ( C 2; ω, L, R 2 ) f ( C; ω, L, R ):= 1 + ω 2 C { C R 2 + L ( ω 2 L C  2 ) } ω 2 L { 1  ω 2 C ( L  C R 2 ) }
 the second capacitor is the fixed capacitor, and
 the optimal curved line is expressed by a formula defined as:
 where C1 is the capacitance value of the first capacitor, C2 is the capacitance value of the second capacitor, L is the selfinductance of the main loop and the amplification loop, co is an angular frequency of a signal to be applied to the main loop, and R2 is the resistance value of the second resistor.
4. The loop antenna according to claim 1, wherein
 the second capacitor is the fixed capacitor, and
 the optimal straight line is a straight line passing through the optimal point and having a slope equal to −1.
5. The loop antenna according to claim 2, wherein
 the second capacitor is a variable capacitor, and
 the capacitance value of the variable capacitor is adjusted to a value obtained by using the capacitance value of the fixed capacitor and the optimal curved line.
6. The loop antenna according to claim 1, wherein
 both of the first capacitor and the second capacitor are the fixed capacitors, and
 the combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor is a combination in which a distance from a point indicated with the capacitance value of the first capacitor and the capacitance value of the second capacitor to any of the optimal curved line and the optimal straight line is shortest.
7. The loop antenna according to claim 1, wherein the capacitance value of the first capacitor and the capacitance value of the second capacitor are values on any of the optimal curved line and the optimal straight lines.
8. A design method for a loop antenna provided with
 a main loop being an open loop connected to any of a signal source and a reception circuit,
 an amplification loop being a closed loop having the same shape as the main loop,
 a first resistor connected in series to the main loop,
 a first capacitor connected in series to the main loop,
 a second resistor connected in series to the amplification loop, and
 a second capacitor connected in series to the amplification loop, wherein
 the main loop and the amplification loop have equal selfinductance,
 a resistance value of the first resistor is a larger value than a resistance value of the second resistor,
 at least one of the first capacitor and the second capacitor is a fixed capacitor,
 a magnitude of a current flowing on the amplification loop is expressed by using the resistance value of the first resistor, the resistance value of the second resistor, a capacitance value of the first capacitor, a capacitance value of the second capacitor, and the selfinductance,
 a combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor to maximize the magnitude of the current is expressed by any of an optimal curved line and an optimal straight line each of which passes through an optimal point indicated with the capacitance value of the first capacitor and the capacitance value of the second capacitor when the magnitude of the current is maximized in orthogonal coordinates adopting the capacitance value of the first capacitor and the capacitance value of the second capacitor as respective axes, and
 the method includes determining the combination of the capacitance value of the first capacitor and the capacitance value of the second capacitor based on any of the optimal curved line and the optimal straight line.
9. The loop antenna according to claim 3, wherein
 the first capacitor is a variable capacitor, and
 the capacitance value of the variable capacitor is adjusted to a value obtained by using the capacitance value of the fixed capacitor and the optimal curved line.
10. The loop antenna according to claim 4, wherein
 the first capacitor is a variable capacitor, and
 the capacitance value of the variable capacitor is adjusted to a value obtained by using the capacitance value of the fixed capacitor and the optimal straight line.
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Type: Grant
Filed: Jun 8, 2018
Date of Patent: Jul 21, 2020
Patent Publication Number: 20200203832
Assignee: Nippon Telegraph and Telephone Corporation
Inventors: Aiichiro Sasaki (Atsugi), Junichi Kodate (Atsugi)
Primary Examiner: Andrea Lindgren Baltzell
Application Number: 16/620,965
International Classification: H01Q 7/08 (20060101); H04B 5/02 (20060101); H01Q 19/02 (20060101);